Rheological properties of enzymatic milk gel: Effect of ion partitioning in casein micelles

Casein micelles (CMs) are colloids of ∼150 nm diameter formed by the association of milk proteins and minerals. Divalent milk cations, i.e. Ca²⁺ and Mg²⁺, exist in two forms in CMs: either directly bound to casein residues (C-bound) without inorganic phosphate, or in a precipitated form called micellar calcium phosphate (MCP). The composition and properties of CMs vary with changes of the physicochemical conditions of milk; e.g. after the addition of salt or a change of pH. Moreover, the rheological properties of enzymatic milk gels are known to be strongly influenced by variations of mineral equilibria. We report in this study on the effect of cation partitioning in CMs on the rheology of enzymatic milk gel. Firstly, salts and acid (CaCl₂, MgCl₂, Na₂HPO₄, Na₃Cit and HCl) were added to produce a set of 19 milk samples containing different proportions of C-bound and MCP in order to identify correlations with rheological parameters (G′, G″, tanδ) of the resulting gels. Three milks were then selected according to the mineral content of CMs and the resulting gels were further characterized in order to propose mechanisms that might explain the dependence of gel macroscopic properties on those of CMs. We showed that the rheological properties of gel were well correlated with the two types of cations present in CMs: an increase in C-bound cations raised the values of G′ and G″ while an increase in the MCP content lowered the tanδ. To explain these inter-dependencies, CMs were viewed as soft colloids whose modulus could be tuned by their MCP content. Consequently, CMs with different mineral contents would have differing abilities to fuse after aggregation, which appeared to link the rheological and microscopic attributes of the gels in a consistent manner. These results will be useful to understand the complex variations in the rheological properties of milk gels produced in the dairy industry.